DRILLING ENGINEERING: HOW DO DRILLING FLUIDS WORK?


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INTRODUCTION


    Drilling fluids, also referred to as drilling mud, are added to the wellbore to facilitate the drilling process by suspending cuttings, controlling pressure, stabilizing exposed rock, providing buoyancy, and cooling and lubricating.

Cuttings in Circulating Driling Fluids Source: Oil & Gas UK

    As early as the third century BC, the Chinese were using drilling fluids, in the form of water, to help permeate the ground when drilling for hydrocarbons. The term "mud" was coined when at Spindletop in the US, drillers ran a herd of cattle through a watered-down field and used the resulting mud to lubricate the drill.
    While the technology and chemistry of drilling fluids have become much more complex, the concept has remained the same. Drilling fluids are essential to drilling success, both maximizing recovery and minimizing the amount of time it takes to achieve first oil.

Purposes of Drilling Fluid

    During drilling, cuttings are obviously created, but they do not usually pose a problem until drilling stops because a drillbit requires replacement or another problem. When this happens, and drilling fluids are not used, the cuttings then fill the hole again. Drilling fluids are used as a suspension tool to keep this from happening. The viscosity of the drilling fluid increases when movement decreases, allowing the fluid to have a liquid consistency when drilling is occurring and then turn into a more solid substance when drilling has stopped. Cuttings are then suspended in the well until the drill is again inserted. This gel-like substance then transforms again into a liquid when drilling starts back up.
    Drilling fluids also help to control pressure in a well by offsetting the pressure of the hydrocarbons and the rock formations. Weighing agents are added to the drilling fluids to increase its density and, therefore, its pressure on the walls of the well.
    Another important function of drilling fluids is rock stabilization. Special additives are used to ensure that the drilling fluid is not absorbed by the rock formation in the well and that the pores of the rock formation are not clogged.
    The longer the well, the more drill pipe is needed to drill the well. This amount of drill pipe gets heavy, and the drilling fluid adds buoyancy, reducing stress. Additionally, drilling fluid helps to reduce friction with the rock formation, reducing heat. This lubrication and cooling helps to prolong the life of the drillbit.

Drilling Fluids Source: OSHA

Types of Drilling Fluids

    Drilling fluids are water-, oil- or synthetic-based, and each composition provides different solutions in the well. If rock formation is composed of salt or clay, proper action must be taken for the drilling fluids to be effective. In fact, a drilling fluid engineer oversees the drilling, adding drilling fluid additives throughout the process to achieve more buoyancy or minimize friction, whatever the need may be.
    In addition to considering the chemical composition and properties of the well, a drilling fluid engineer must also take environmental impact into account when prescribing the type of drilling fluid necessary in a well. Oil-based drilling fluids may work better with a saltier rock. Water-based drilling fluids are generally considered to affect the environment less during offshore drilling.
    Disposal of drilling fluids after they are used can also be a challenge. Recent technological advances have established methods for recycling drilling fluids.